The economic value of gemstones, particularly diamonds, is highly dependent upon the color of such stones. Within each type of gemstone category, there is a range of color variation which affects value. For diamonds, color variation typically extends from colorless to yellow. Colorless diamonds command the highest prices and may run into thousands of dollars per carat. Diamonds containing a typical yellow tint may cost only a fraction of that of a colorless diamond although a true canary yellow diamond may be relatively expensive.
The presently accepted method for color grading of precious gemstones, such as diamonds, requires comparison of a stone to be graded with stones in a set of certified standard gemstones which may be actual diamonds or synthetic diamonds of cubic zirconium. This method requires extensive training and observations in a standardized light environment. Color grading then requires seeking of a match between one of the stones in the set and the sample stone being graded. The cost of such standardized sets of gemstones precludes their use in many instances.
A further disadvantage of the prior art method of color grading is the inconsistency which routinely appears between trained appraisers and gemologists. Diamonds are typically graded with three separate grades of what is recognized as colorless, i.e., D, E, and F grades are all colorless but there is a slight degradation in progressing from the highest grade D to the lower grade F. Similarly, a near colorless diamond may be graded as G, H, or I grade. Below near colorless grade are size grades ranging from J to O which have varying degrees of yellow tints. If any one of these grades of diamonds are examined without reference to the others, it is virtually impossible to determine the diamond color grade. Even when examined as part of a standard set, differences in lighting may affect the appraiser's ability to accurately grade a diamond.